A government review panel warned Thursday that the National Security Agency's daily collection of Americans' phone records is illegal and recommended that President Barack Obama abandon the program and destroy the hundreds of millions of phone records it has already collected.

The panel's 234-page report included dissents from two of the board's five members — former Bush administration national security lawyers who recommended that the government keep collecting the phone records. The board described key parts of its report to Obama earlier this month before he announced his plans last week to change the government's surveillance activities.

In that speech, Obama said the bulk phone collection program would continue for the time being. He directed the Justice Department and intelligence officials to find ways to end the government's control over the phone data. He also insisting on close supervision by a secretive federal intelligence court and reducing the breadth of phone records the NSA can investigate. Phone companies have said they do not want to take responsibility for overseeing the data under standards set by the NSA.

In addition to concluding that the daily collection of phone records was illegal, the board also determined that the practice was ineffective.

"We have not identified a single instance involving a threat to the United States in which the program made a concrete difference in the outcome of a counterterrorism investigation," it said, and added, "We are aware of no instance in which the program directly contributed to the discovery of a previously unknown terrorist plot or the disruption of a terrorist attack."

It said the NSA should instead seek individual records relevant to terror cases directly from phone service providers under existing laws.

"Given the limited results, we concluded the program should be ended," said board member James Dempsey of the Center for Democracy and Technology, a civil liberties group. Dempsey and another board member, former chief federal appeals court judge Patricia Wald, also said the phone sweeps did not appear to have clear or strong legal grounding in the USA Patriot Act — the statute overseeing the government's surveillance activities.

The board wrote that the phone surveillance did not have a "viable legal foundation" under the Patriot Act, which was used to provide legal backing for the operation after it was secretly authorized by President George W. Bush. The board also said the surveillance raised constitutional concerns about unreasonable searches, free speech and freedom of the press. Two federal judges have split in recent rulings over the constitutionality of the government collecting Americans' phone records in such a wholesale way.

The White House disagreed with the oversight board. "The administration believes the program is lawful," said national security spokeswoman Caitlin Hayden. She added that Obama "believes we can and should make changes in the program that will give the American people greater confidence in it."

The board's recommendation to delete its copies of everyone's phone records is one area that Obama sidestepped in his speech — what to do with the mountainous electronic database amassed by the NSA since shortly after the 9/11 attacks.

The NSA's surveillance programs and other data mining operations came to light last year, drawing intense criticism after revelations fueled by an estimated 1.7 million documents taken by former NSA systems analyst Edward Snowden and handed over to several journalists.